Global pollutant concentrations in coal mine soils: Discussing an approach to the meta-study

The extraction of solid fossil fuels results in the accumulation of overburden and host rocks stored on the Earth's surface. Coal mining sites are among the most disturbed and polluted areas. Soils are affected by these transformations in multiple ways, including structural changes, the loss or suppression of vegetation cover, and the migration and accumulation of chemical elements in soils and water. To assess the global concentrations of chemical elements in the coal mine soils, we discussed and developed a meta-study on pollutants in Technosols and altered natural soils. For this, we collected data from papers published in peer-reviewed journals between 2000 and 2022, covering 25 major coal-producing countries of Eurasia, Africa, Australia, and the Americas. To understand better the patterns of soil pollution driven by coal extraction itself, we gathered the concentrations measured in soils, spoils, and dumps near open-cut and underground coal mines. For the same reason, the data on pollutants in remediated or reclaimed soils, as well as in soils near coal power plants (or other pollution sources) were excluded. Likewise, we did not consider other abiotic (e.g., coal ash, mine water) or biotic media (e.g., grasses, trees, and plants in general)  even though they are undoubtedly interlinked. Moreover, the data on soil pollution are far more abundant and thus statistically significant.

The typical set of keywords used for searching in databases included “coal mine”, “soil/dumps”, “pollution/contamination”, and “elements/metals”. Obviously, other terms like “colliery”, or “wasterock”, or “geochemical transformation” were applied too but gave fewer search results. To harmonize measurement units, we recalculated all data to mg/kg or ppm. When necessary, concentrations were recalculated from oxides into elemental forms. To confirm the representativeness of the figures, we checked the number of specimens analyzed in each research. The total number of samples used in the meta-study was over 7,000. For the standard statistical processing, the mean concentrations were collected alongside the minimum and maximum contents, and standard deviation values; when not provided in a paper, they were calculated from the raw data. After that, we obtained the average contents of chemical elements that characterize each coalfield separately.

The preliminary results reveal that priority pollutants are inherited from the world averages for trace element contents in coals rather than the natural background. In other words, concentrations of priority pollutants are predominantly determined by coal extraction and the release of related pollutants. The research outcomes indicate that the oxidation-reduction and alkaline-acid milieu, water and temperature regimes, sorption capacity, and other landscape-geochemical conditions are being ambiguously transformed in new ecosystems and can be derived from both natural conditions and the man-inflicted damage. The geochemical cycles in biocenoses are altered and the tasks for their restoration may vary significantly. The established global concentrations of chemical elements in coal mine soils can be used for comparative assessments and the management of legacy contamination and soil/landscape rehabilitation in post-mining regions. However, remediation efforts will also need to consider site-specific geological, hydrological, and climatic characteristics as well as socio-economic conditions and other regional development objectives.